This invention relates to circuits for driving gas discharge lamps, and particularly, though not exclusively, to circuits for driving fluorescent lamps.
In a typical prior art circuit for driving a plurality of fluorescent lamps, the lamps are driven from a high-frequency oscillating circuit powered, via a rectifier and an inverter, from an AC voltage supply, e.g. an electric utility mains.
In one such typical prior art circuit the high-frequency oscillating circuit is based upon an inductance and a capacitance coupled in series to form a series-resonant combination, and the inverter is based upon two transistor switches connected in a half-bridge configuration.
Typically, in use of such a circuit, a fluorescent lamp load is connected in parallel with the high-frequency oscillating circuit, i.e., in parallel with both the capacitance and the inductance. However, in a modification of this arrangement the fluorescent lamp load may alternatively be connected in parallel with the capacitance but in series with the inductance. Such a modified arrangement is particularly suited to driving gas discharge lamps such as fluorescent lamps which have very pronounced non-linear dynamic characteristics.
In such a modified circuit, the power transferred to the load decreases as the frequency of the circuit increases for a given load, and increases as the load impedance increases for a given working frequency. It is possible to effect controlled dimming of fluorescent lamps driven from such a modified circuit by controlling the circuit's operating frequency in order to control the power transferred to the load. However such a method of controlled dimming suffers several fundamental drawbacks:
Firstly, great care needs to be taken in order to avoid the possibility of the circuit's frequency falling below a critical frequency at which the circuit begins to oscillate in a "capacitive" mode (i.e., with a negative phase angle). Such a mode of oscillation causes transverse cross-conduction currents to flow through the half-bridge switching transistors, leading to their eventual destruction because of the excess power dissipation caused by the cross-conduction currents. This problem is not easy to avoid satisfactorily, since it is otherwise desirable for the circuit to operate near to this critical frequency in order to deliver the highest power to the load at the highest efficiency.
Secondly, the efficiency of the circuit over the range of dimming is compromised. For cost reasons, the circuit is typically designed to deliver the maximum power at the maximum efficiency level, thus reducing the constraints on the sizes of the magnetic elements of the circuit and on the switching transistors which optimally operate close to zero-current switching levels. Once the circuit's frequency increases in order to perform dimming, the transistors' current switching angle increases, forcing the transistors to switch farther away from the zero-current level. Also, the circulating reactive current in the circuit first increases before decreasing, creating a much higher power loss in the circuit over a significant portion of the frequency range. In order to accommodate this increased power loss, the magnetic elements and the switching transistors have to be re-designed with greater tolerances than would otherwise be required.
Thirdly, for a given desired range of dimming, the required range of frequency variation is proportionately greater, due to the non-linear behavior of the fluorescent lamp load. Gas discharge lamps such as fluorescent lamps are well-recognized as presenting a negative impedance over a significant part of their impedance spectrum. Thus, over the negative impedance range, whenever lamp current decreases lamp voltage increases (though at a lower rate), leading to an increase in the equivalent load impedance which makes the circuit draw more power. This behavior runs counter to the objective of dimming by frequency control, over at least a part of the range of frequency variation, and so necessitates a much greater frequency control range in order to accomplish a desired range of dimming.